All the email sent by OsClass have formatting problem
They have HTML tags but they are as text in the message body ... As if they hadn't been interpreted
Header issue ?
Note that I run PHP 8.0

This looks like your mail server sends HTML as text.
Or your client blocks html rendering?
Try to reconfigure it.

May be a problem with PHP 8 ?
https://github.com/joomla/joomla-cms/issues/32830 "PHPMailer creates wrong mail headers with PHP 8"
very similar issue

@philoo
Maybe, you are first one complaining, for me it worked OK.

With PHP 8 ??

@philoo
Many people already use PHP 8.0, it was supported from Osclass 4.3 released in January 2021.

Will check with server SysOp ....
Thank you

Same issue

PHP 8.1 sending html format

PHP 7 working correctly

Osclass does not support PHP 8.1, libraries have dependencies and if we want to keep wide range for php versions supporting osclass, we need to wait a bit with 8.1.
Osclass sends HTML format.

Osclass does not support PHP 8.1, libraries have dependencies and if we want to keep wide range for php versions supporting osclass, we need to wait a bit with 8.1.
Osclass sends HTML format.

Change PHP version 8.1  to 8

same result

<p>Hi {CONTACT_NAME}!</p>
<p>{USER_NAME} ({USER_PHONE}) left you a message about your item {ITEM_TITLE}:</p>
<p>{COMMENT}</p>
<p>Regards,</p>
<p>{WEB_TITLE}</p>

It's ok, only problem is that you are having HTML as text, but do not think this can be configured in Osclass at all.
Osclass send mail as HTML format, not plain text. What does your mail server or PHP mailer class with it is different question.

It's ok, only problem is that you are having HTML as text, but do not think this can be configured in Osclass at all.
Osclass send mail as HTML format, not plain text. What does your mail server or PHP mailer class with it is different question.

i don't understand   

Try to use different mail server. I am not able to reproduce anything like that.

I got the same error when I was using GoDaddy hosting. Also, GoDaddy doesn't let you send the mail from another SMTP server. They force you to use their own Relay server to send the mail. I recently switch to Hostinger, mail is now sent in HTML format. Also, there is no restriction on sending mail through SMTP.

@Jhom
Thanks for your feedback!